1. Field of the Invention
The present invention relates to a method of providing a set of alignment marks on a substrate, to a device manufacturing method and to a lithographic apparatus. The invention also relates to a method of determining the relative displacement of at least two layers provided with the alignment marks.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In general, the wafer is covered with a plurality of layers, placed on top of each other. In the layers patterns are exposed, wherein the pattern of a layer has to fit exactly on the previous one. In practice, there will always be an offset between consecutive layers. The offset or displacement of the layers relative to one another is referred to herein as the overlay error or overlay. The overlay is determined by one or more alignment sensors arranged to measure the positions of substrate alignment marks arranged for that purpose on the layers.
Several methods exist to determine the overlay capability of a lithographic apparatus. First, use can be made of one or more of the alignment sensors typically present in the lithographic apparatus. The alignment sensor measures the relative position of an alignment mark in a first layer with respect to the position of the same or similar alignment mark in a second layer, placed on top of the first layer. However, the overlay measurements rely on alignment marks or similar overlay targets with dimensions that are much larger than the typical product features to be provided on the substrate. Furthermore, the alignment sensors are designed for an accuracy that is much larger than the typical product features as well.
Alternatively, use may be made of separate metrology tool. For instance, the location of the outer box/frame in the first layer with respect to the location of the inner box in the second layer may be determined with the metrology tool. This method is relatively slow and involves removing the substrate from the lithographic apparatus after having provided the first layer on the substrate. Finally, the measurement of the overlay may be SEM tool based. The location of a set of lines in a first layer may be measured at resolution (i.e., with an accuracy having the same order of magnitude of the typical product features in the layer) with respect to the location of a second set of lines at resolution in a second layer. However, also this SEM tool based method is comparatively time consuming and complex (or even impossible) to accomplish when the first layer target is located in a product layer.